There's no strict answer to that question, depends on lots of factors. The often-quoted 1.3 seconds for the R22 isn't strictly true, depends on airspeed/alt/power in use/weight/temp/rpm, it also depends on what else you do when you lower the lever, ie hauling the cyclic back and standing the R22 on it's tail (as long as you have at least 60kts) will restore the RPM in the green pretty sharpish.